Water leaves the storage cistern in distribution pipes which are usually 22mm (3/41n) or 15mm (1/2in) in diameter. In a direct system, supply from the cistern will usually only be to the hot water tank, and in an indirect system this link must also be direct — but other distribution pipes are used with branches to supply the other appliances — basins, baths and WC cisterns. Distribution pipes usually end in taps but in the case of a WC a low pressure ball-valve controls the flow.

The WC in an indirect system has a low pressure ball-valve because when the water leaves the storage cistern it is no longer at mains pressure but at normal atmospheric pressure which is pressing down on the surface of the stored water. This means that the higher up the house a tap or other outlet is situated the lower will be the water pressure. In practice this means that you can’t have a tap in an indirect system which is above the level of its distribution outlet from the cistern. Showers are particularly affected by this difference of pressure, and if there is not sufficient ‘head to ‘drive’ the shower a special pump may have to be installed. Cold water supplied to the hot water tank is heated in two different ways again called indirect and direct systems — or, respectively, closed and open. In the latter the cold water is circulated through the boiler, where it is heated, and returned to the tank from where it flows to tapped outlets. In the indirect system the cold water supplied never actually goes to the boiler, instead it is heated in the tank by a coiled pipe or jacket containing hot water which is continuously circulating through the boiler. In either case a pump often helps the water flow through the boiler, and supplementary or alternative heat may come from an immersion heater. If there is no boiler but only an immersion heater in the tank the system is essentially direct with the heating of the water taking place in the tank rather than in the boiler.

Draining the system Just above the rising main stop-valve should be a drain cock. With the stop-valve turned off the drain cock can be used to drain part of the cold water system when repairs are necessary — the hot water system has its own drain cock.